1. Field of the Invention
This invention relates to radio position determination systems. In particular, the invention relates to geo-referencing the phase centers of antennas of different radio positioning systems that are being concurrently used.
2. Description of Related Art
The Global Positioning System (GPS) is a type of radio position determination system. The GPS is designed to provide users with position solutions on a worldwide, 24-hour-per day basis. However, there are occasions when the GPS by itself is not sufficient for a given task. For example, when the user's view of the sky is limited due to trees or other objects, the user may not be able to see a sufficient number of satellites for obtaining a position solution. For these and other reasons, it is desirable to augment the GPS with auxiliary positioning signals that use radio determination principles.
Whenever there is more than one radio determination system being used the position of all the antennas must be known in a common reference coordinate system (datum). Ranging information or lines of position from sources having different datums have to have the datums transformed to a common datum before they become usable. Consequently when lines of positions from auxiliary sources are used with lines of positions from GPS satellites the antenna phase centers of the auxiliary sources must be known in the datum of the GPS or WGS-84.
These auxiliary ranging signals may take several forms. For example they could be signals from a ground based transmitter which transmits GPS-like signals. Such a system is termed a pseudolite. With a pseudolite, the GPS-like signals could be directly tracked by a GPS receiver, thereby providing an extra line of position to the GPS receiver. There are forms other that pseudolites that can be used to augment GPS.
Augmentation of one radio determination system by another does not have to include GPS. For example, a Del Norte Trisponder system could be used to augment a Sercel Syledis system. In this case the location of the Trisponder antennas would have to be known in the same datum as that used by the Syledis.
As stated, if the GPS system is to be augmented by another radio determination system the auxiliary system will have to be referenced to the same geodetic datum as the GPS for it to be usable. For ground based transmitters this entails placing the antennas of the auxiliary system at a surveyed points whose coordinates are referenced to the GPS datum, the WGS-84.
For satellite or other unstable platforms having the antenna of an auxiliary system to be used with the GPS, the auxiliary antennas have to be radio positioned in real time with the coordinates of the positioning system known in the WGS-84 datum.
There are several ways to reference or know the location of an antenna providing auxiliary signals. It can be referenced to a local grid on a local datum, to a local grid on a local tangent plane or to any number of datums, grids and projections including the WGS-84 datum. There are several well-known survey methods for establishing the point of reference of the auxiliary antenna but the frame of reference of the phase center of the auxiliary antenna must be in the same datum as the phase center of the antenna of the system it is augmenting.
If the auxiliary antenna is to be a permanent installation then it is possible to use conventional techniques to establish its location in a datum including the WGS-84. Unfortunately, there are many cases where the location of the auxiliary antenna will be of an ad hoc nature. For example in an open pit mine the location of the auxiliary antennas could be expected to change on a weekly basis. For these cases the repeated surveying in of antenna locations becomes a costly chore.
Similarly, for situations where the auxiliary antenna is on an unstable platform, such as a satellite, there are well-known but costly techniques of determining, in real-time, the location of the antenna on such a platform.
Therefore there is a need in the technology to provide a simple and unambiguous method to determine the positions of the phase centers of antennas which augment radio determination systems, such as those augmenting the GPS, within the same datum used by the system being augmented.